1. Field of the Invention
The invention relates to a cuvette for measuring the absorption of irradiation in liquid samples.
2. Description of the Prior Art
The cuvettes to be taken into account here are used applied in absorption photometers which measure the spectral transmission factor, the spectral retransmission factor or the spectral absorption measure (extinction) of a sample. From the measurement of these values conclusions may be made as to the qualitative or quantitative composition of the sample.
Photometers function in a certain wavelength region, whose selection is effected by color or interference filters. With the spectral photometers on the other hand the light of the illumination means is spectrally disaggregated with a monochromator. With calorimeters the determination of the concentration is effected by color comparison with a standard solution of the same substance. The irradiation may in particular lie in the region of the visible light, in the IR or UV region. The selection may be dependent of the sample to be analyzed. The material selection of the cuvette is in turn directed to the spectral region for which this is to be used.
From DE 17 73 233 C3 there is known a vessel for the optical examination of small fluid quantities which has a lower part with a cross section rectangular on the inside and optically transparent walls and an upper part with larger diagonals than the lower part, wherein the upper part is designed cylindrically. The vessel is closed below and open at the top and at the outside on the lower rectangular part there are attached perpendicular ribs, which extend to the outer dimension of the upper part, wherein the ribs end at a covering on an imagined enveloping cylinder of the upper part. The vessel is suitable as a measuring cuvette and also as a reaction vessel for manual or mechanized sample preparation. The rectangular cross section of the lower vessel part permits two different layer thicknesses for the photometric measurement. The ratio of the layer thicknesses is about 2:1. With this cuvette there is still required a considerable fluid quantity in order to sufficiently fill the lower part for the photometric measurement. Furthermore the stability of the cuvette is not particularly good, since it tapers downwards. Finally there is only a slight variability of the layer thickness.
For the UV region until now cuvettes of quartz glass have been used. These are very expensive and on account of the danger of contamination by the previous sample, on account of the required cleaning work and by way of the danger of breakage they are more unfavorable in the handling.
Indeed for examinations in the UV range often only the smallest of sample quantities are available, without it being known whether these are present in a concentration suitable for a measurement. An example is the DNA photometry with which by measurement in the UV range the ratio of DNA/RNA, the presence of proteins and the opaqueness of a sample are measured. With this it is the case of a routine examination in the genome analysis, for which many times only a small quantity may be branched off. This must be diluted many times in order to fill a cuvette sufficiently with regard to measuring technology. As a result of this the concentration of the sample may be very different. Only after filling into a cuvette may it then be ascertained that with the given concentration and layer thickness a photometric measurement is not possible. Then the sample must be removed, diluted again, again filled into a cuvette and examined. The dilution may however also be so great that the photometric measurement lies on the limit of proof and thus is very erroneous.
Standard cuvettes have a quadratic cross section with outer dimensions of 12.5.times.12.5 mm. Absorption photometers have a cavity with a corresponding cross section, into which the cuvettes may be inserted. Transversely through this cuvette cavity runs the beam path of the illumination means. Absorption photometers of various manufacturers differ by the distance of the beam path to the floor of the cuvette cavity. Also the irradiation with adsorption photometers of various manufacturers has various cross sections. Until now the cuvette geometries are adapted to the various apparatus types. The use of a single cuvette type for small measuring volumes with various apparatus types is not possible at present.
Furthermore the known plastic cuvettes have a flat bearing in the cuvette cavity. Thus it is difficult to manufacture the cuvettes such they have an exact positioning in the cuvette cavity and do not wobble or jam therein.
Proceeding from this it is the object of the invention to provide a cuvette which favors the adsorption-photometric measurement of the smallest sample quantities of differing concentrations and has an increased stability. Further it is the object of the invention to provide a cuvette which with a simple manufacturability permits a more exact positioning in the cuvette cavity. Finally the object of being able to use a cuvette in absorption photometers with a various arrangement and formation of the beam path is to be achieved.